1. Field of the Invention
The present invention relates to a microspectroscope for detecting the spectrum of observed light from a sample to be measured.
2. Description of the Prior Art
FIG. 1 is a schematic block diagram showing a conventional microspectroscope 1. As shown in FIG. 1, the microspectroscope 1 comprises an illuminating optical system 10, a microscopic optical system 20, a reflecting mirror 30, a spectroscopic unit 40, and a monitoring optical system 50.
The illuminating optical system 10 is formed by a light source 11, a condenser lens 12, an aperture stop 13, a field stop 14 and another condenser lens 15, so that illuminating light from the light source is guided to the microscopic optical system 20 through the condenser lens 12, the aperture stop 13, the field stop 14 and the condenser lens 15.
The microscopic optical system 20 is formed by an objective lens 21, an image-formation lens 24 and a beam splitter 23 provided between the objective lens 21 and the image-formation lens 24. Symbol 22 denotes a pupil position of the objective lens 21.
The illuminating light from the light source 11 passes through the condenser lens 12, the aperture stop 13, the field stop 14 and the condenser lens 15, and is guided to the objective lens 21 by the beam splitter 23. The illuminating light transmitted through the objective lens 21 is applied onto the surface of a sample S which is supported by a sample holder (not shown).
Reflected light reflected by the surface of the sample S is directed through the objective lens 21, the beam splitter 23 and the image-formation lens 24 to be enlarged and imaged at a location close to the reflecting mirror 30.
The reflecting mirror 30 is provided with a pinhole 31. Within the reflected light, therefore, reflected light L.sub.S (which passes through the pinhole 31) is received by the spectroscopic unit 40.
The spectroscopic unit 40 is formed by a diffraction grating 41 for separating the reflected light L.sub.S into spectral components and a photo detector 42 for detecting the spectrum of the light spectrally diffracted by the diffraction grating 41. The diffraction grating 41 may be a flat field type diffraction grating which images a spectrum on a flat plane, for example. Alternatively, the diffraction grating 41 may have a sweeper. The photo detector 42, which is formed by a photodiode array or a CCD, for example, is conjugate with the pinhole 31. Alternatively, the photo detector 42 may be a photomultiplier.
The reflected light L.sub.S received by the spectroscopic unit 40 is separated into its spectral components by the diffraction grating 41, and the respective spectral components of the light L.sub.S are received by the photo detector 42, which in turn outputs a signal corresponding to the spectrum of the light L.sub.S.
The reflected light that is reflected by the reflecting mirror 30 enters the monitoring optical system 50, and is imaged on an image-formation position 52 through a relay lens 51. Thus, an enlarged image of the surface of the sample S is imaged on an image-formation plane, so that the measuring position of the sample S can be confirmed and focusing can be performed on the basis of the enlarged image.
When illuminating light is applied onto the surface of a sample S which comprises a substrate and a transparent thin film formed thereOn, such as a silicon substrate and a silicon oxide film, for example, light reflected by the surface of the thin film and light transmitted through the thin film and then reflected by the surface of the substrate interfers with each other within the microspectroscope. The degree of such interference depends on the indexes of refraction of the substrate and the thin film, the thickness of the thin film and the wavelength of the illuminating light.
Since the indexes of refraction of the substrate and the thin film and the wavelength of the illuminating light are constant the degree of interference of the reflected light depends solely on the thickness of the thin film. The microspectroscope 1 outputs a detection signal relating to the spectrum which is responsive to the thickness of the thin film. Therefore, the conventional microspectroscope 1 is generally useful within to a film thickness measuring apparatus.
As shown in FIG. 1, such a film thickness measuring apparatus 2 is formed by the microspectroscope 1 and an arithmetic unit 3, so that the microspectroscope 1 detects the spectrum of the sample S and the arithmetic unit 3 computes the film thickness of the sample S on the basis of spectral data obtained by the microspectroscope 1.
The spectrum detected by the microspectroscope 1 is influenced by various factors such as spectral transmittance characteristics of the illuminating optical system 10 and and the microscopic optical system 20, luminous energy loss caused when the light passes through these optical systems, the spectral characteristic of the diffraction grating 41, the spectral-response characteristic of the photo detector 42, and the like. In order to accurately measure the film thickness of the sample S, it is necessary to eliminate such influences.
Thus, errors caused by such factors are calibrated as follows:
FIG. 2 is a flow chart showing a method of measuring film thickness by the film thickness measuring apparatus 2. Prior to measurement, an operator inputs spectrum data B(.lambda.) of a sample (hereinafter referred to as "standard sample") in the arithmetic unit 3 through a keyboard (not shown). The spectrum data of the standard sample is known in the data. The known is stored in a memory (not shown) provided in the arithmetic unit 3. The standard sample may be a silicon substrate, a substrate which is deposited with aluminum on its surface, or the like.
Then the operator sets the standard sample on the sample holder of the microspectroscope (step S1), and supplies a command signal for detecting calibration data to the arithmetic unit 3. In response to a command from the arithmetic unit 3, the microspectroscope detects the spectrum of the standard sample and stores data B'(.lambda.) relating to the spectrum in the memory of the arithmetic unit 3 (step S2).
Then, the operator removes the standard sample from the sample holder of the microspectroscope 1 and sets the sample S on the sample holder of the microspectroscope 1 (step S3). Thereafter the operator supplies a command to the arithmetic unit 3 for starting measurement, so that the microspectroscope 1 detects the spectrum of the sample S in response to a command outputted from the arithmetic unit 3, and data S'(.lambda.) relating to the spectrum of the sample S is stored in the memory of the arithmetic unit 3 (step S4).
At a step S5, the data S'(.lambda.), B(.lambda.) and B'(.lambda.) stored in the memory are read in the arithmetic unit 3, to obtain data S(.lambda.) in accordance with the following expression: ##EQU1##
The data S(.lambda.) corresponds to a signal outputted from the microspectroscope 1 on the assumption that absolutely no influence is caused by the aforementioned factors. In other words, the data S(.lambda.) shows the true spectrum of the sample S.
Using the data S(.lambda.) the arithmetic unit 3 comprises the thickness of the thin film (step S6). This computation is itself well known and hence description thereof is omitted.
As understood from the expression (1), the data S'(.lambda.) relating to the actually measured spectrum is calibrated to obtain the data S(.lambda.) relating to the true spectrum, whereby the film thickness can be accurately measured.
Further, since the aforementioned factors are not influenced by peripheral environmental changes around the apparatus 2 such as temperature, humidity etc. but remain constant, the data B'(.lambda.) once measured will not be significantly changed. Therefore, the steps S3 to S6 are continuously repeated to measure the thickness of a subsequent sample S. to thereby continuously accurately measure the film thickness.
However, the spectrum also influenced by factors other than the above, such, as, spectral emissivity which is change with ambient temperature of the light source 11, for example. When the ambient temperature of the light source 11 changes, spectral emissivity changes accordingly, to vary the spectrum actually measured by the microspectroscope 1. It is assumed here that the light source 11 has a certain spectral emissivity characteristic (hereinafter referred to as "first characteristic") in measurement of the data B'(.lambda.) at the step S2, for example When the characteristic of the light source 11 affecting detection of the spectrum of the sample S is substantially identical to the first characteristic, the film thickness can be measured with no particular problem.
However, when a plurality of samples S are continuously measured and the light source 11 has a second characteristic which differs significantly from the first characteristic after a certain period of time, measurement accuracy is reduced. This is because the data S'(.lambda.) is detected by illuminating light from the light source 11 which has the second characteristic, although the data B'(.lambda.) is detected by the illuminating light from the light source 11 which has the first characteristic.
To continuously maintain correct measurement, therefore, it is necessary to frequently measure the data B'(.lambda.) after measuring the film thickness of the sample S, as shown by dotted lines in FIG. 2. In this case, efficiency is reduced since the standard sample must be replaced on the sample holder of the microspectroscope 1.